Thermotropic liquid crystals are generally crystalline compounds with significant anisotropy in shape. That is, at the molecular level, they are characterized by a rod-like or disc like structure. When heated they typically melt in a stepwise manner, exhibiting one or more thermal transitions from a crystal to a final isotropic phase. The intermediate phases, known as mesophases, can include several types of smectic phases wherein the molecules are generally confined to layers; and a nematic phase wherein the molecules are aligned parallel to one another with no long range positional order. The liquid crystal phase can be achieved in a heating cycle, or can be arrived at in cooling from an isotropic phase. The structure of liquid crystals in general, and twisted nematic liquid crystals in particular, is further discussed in “The Physics of Liquid Crystals”, de Gennes and Prost, Oxford University Press, 1995.
An important variant of the nematic phase is one wherein a chiral moiety is present, referred to as a twisted nematic or cholesteric phase. In this case, the molecules are parallel to each other as in the nematic phase, but the director of molecules (the average direction of the rodlike molecules) changes direction through the thickness of a layer to provide a helical packing of the nematic molecules. The pitch of the helix is perpendicular to the long axes of the molecules. This helical packing of anisotropic molecules leads to important and characteristic optical properties of twisted nematic phases including circular dichroism, a high degree of rotary power; and the selective reflection of light, including ultraviolet, visible, and near-IR light. Reflection in the visible region leads to brilliantly colored layers. The sense of the helix can either be right-handed or left-handed, and the rotational sense is an important characteristic of the material. The chiral moiety either may be present in the liquid crystalline molecule itself, for instance, as in a cholesteryl ester, or can be added to the nematic phase as a dopant, with induction of the cholesteric phase. This phenomenon is further discussed in sources such as Bassler and Labes, J. Chem. Phys., 52, 631 (1970).
There has been interest in the preparation of stable polymer layers exhibiting fixed nematic and/or cholesteric optical properties. One approach has been to synthesize monofunctional and/or polyfunctional reactive monomers that exhibit a nematic or cholesteric phase upon melting, formulate a low melting liquid crystal composition, and polymerize the liquid crystal composition in its nematic or cholesteric phase to provide a polymer network exhibiting stable optical properties of the nematic or cholesteric phase. This has resulted in the preparation of compositions comprising mixtures of monofunctional and/or polyfunctional reactive monomers, and U.S. Pat. No. 5,833,880, for example, discloses liquid crystalline mixtures comprising at least two different compounds of the following formula
where Z1 and Z2, independently of one another, are polymerizable groups, Y1 and Y2, independently of one another, are each a direct bond, —O—, —COO—, —OCO—, or —S—, A1 and A2, independently of one another, are spacers, and R1, R2, and R3, independently of one another, are as defined in the reference, with the proviso that -A1-Y1— and -A2-Y2—, in at least one of the substances are different O-alkylene radicals.
U.S. Pat. No. 6,090,308 relates to direactive mesogenic compounds or mixtures thereof comprising mesogen-containing molecules, the mesogens having two side chains attached which contain a terminal polymerizable functional group, the mesogens and the functional groups being separated by spacer groups having at least two to twenty spacer atoms, wherein both spacer groups have different chain length.
U.S. Pat. No. 6,136,225 discloses polymerizable liquid crystalline compounds of the Formula Z1-Y1-A1-Y3-M-Y4-A2-Y2-Z2 where Z1 and Z2 are radicals containing reactive groups via which polymerization can be effected, Y1-Y4 are a single chemical bond, oxygen, sulfur, —O—CO—, —CO—O—, —O—CO—O, —CO—NR—, —NR—CO—, —O—CO—NR—, —NR—CO—O— or —NR—CO—NR—, where at least one of the groups Y3 and Y4 is —O—CO—O—, —O—CO—NR—, —NR—CO—O— or —NR—CO—NR—, A1 and A2 are spacers having 2 to 30 carbon atoms in which the carbon chain may be interrupted by ether oxygen, thioether sulfur or by nonadjacent imino or C1-C4-alkylimino groups, M is a mesogenic group, R is C1-C4-alkyl, and compositions comprising them.
US-A-2007/0228326 discloses chemical synthesis of bis(meth)acrylate liquid crystal compounds and polymerization of liquid crystal compositions to provide polymer networks having nematic and cholesteric properties. US-A-2007/0267599 is related to the chemical synthesis of bis(meth)acrylate chiral compounds, liquid crystal compositions comprising the chiral compounds, and polymerization of the liquid crystal compositions to provide polymer networks having cholesteric properties.
A need nevertheless remains for liquid crystal compositions that have broad thermal windows, low melting points and good phase stability against crystallization, and that are easy to prepare and can be tuned to give desired properties.